夜视要注意的点

当您全神贯注地在视力极限下检查一个深空物体时，在10或15秒后是否会变得更难看？天空背景变亮为暗灰色吗？诊断：您屏住呼吸而没有意识到。低氧可以快速杀死夜视。一个老的变星观察者的诀窍是在尝试最暗淡的目标之前要沉重呼吸 15秒左右。并在寻找时保持稳定的呼吸。

酒精，尼古丁和低血糖也会损害夜视，因此在进行深空观察时，请勿喝酒，吸烟或饥饿。带些零食。维生素A的缺乏会损害夜视能力，但是如果您已经摄取了足够的维生素A，那么多摄取维生素A并不会带来任何好处。除非很少食用垃圾食品，否则发达国家中很少有人会摄取维生素A缺乏症，因此不要指望一夸脱的胡萝卜汁改善视力。但是要吃蔬菜。

长时间暴露在明亮的阳光下会降低您几天适应黑暗的能力，因此在户外花费大量时间时要戴墨镜。确保墨镜上的标签说它们阻挡了紫外线（UVA和UVB）；一些便宜的没有。

多年来，紫外线日光（甚至是可见光下的日光）会老化您的晶状体和视网膜，从而降低敏感性并增加退化性疾病（如白内障和黄斑变性）的可能性。因此，如果您在户外佩戴普通眼镜，请要求您的验光师在其上涂紫外线过滤涂层。这种选择既便宜又方便，并且会极大地减少您一生的紫外线暴露，因此无论是否有任何紧急医疗需求，每个眼镜佩戴者都应该使用。

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银河系结构考究

http://www.galaxymap.org/drupal/node/205

马刺和桥梁

img对螺旋星系的仔细检查表明，大多数星系具有复杂的结构，其中包括许多连接主要螺旋臂的短刺或短桥。银河系也是如此，例如在Face On Explorer中可以看到。特别是，在英仙座和射手座之间有几个长而温暖的原子氢刺。其中最长的猎户座骨刺包含太阳。猎户座骨刺起源于射手座臂内或附近，并穿过英仙座臂和诺玛座臂，终止于遥远的外星系。英仙座臂和射手座臂之间的区域包括另外两个大型马刺，小天鹅和维拉马刺，以及许多短桥。Shepherd踢马刺似乎将Norma手臂连接到内部星系的银河环上。

除了这些较大的结构外，“ 探索者面孔”还揭示了外星系中旋臂之间的许多较短的桥梁或“羽毛”。

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OBSERVING THE MILKY WAY, PART I

SkyandTelescope.comJuly 2013 25OBSERVING THE MILKY WAY, PART I: Photo by Babak TafreshiMilky Way Pt1 layout.indd 255/2/13 9:42 AM
26 July 2013sky & telescopeGalactic Depth PerspectiveVital StatisticsProfessional astronomers have nearly as much trouble as amateurs in discerning our galaxy’s structure; it’s diffi -cult to understand a forest when you’re surrounded by its trees. The broad outlines have been known or suspected for many decades, but the details are still controversial.The Milky Way Galaxy seems to be an SBc spiral with fairly loosely wound, luminous spiral arms and a relatively small central bulge. The galaxy’s disk is probably not quite 100,000 light-years in diameter, and the Sun is between 25,000 and 29,000 light-years from the center. The spiral arms are less than 1,500 light-years thick when viewed edge on, but they’re embedded in a thick disk of more smoothly distributed stars some 3,000 to 4,000 light-years thick. The galaxy’s central bulge is something of a fl attened spheroid, with a polar diameter of roughly 8,000 light-years and an equatorial diameter around 10,000 light-years. In both the central bulge and the disk, star densities taper off gradually, so these dimensions cannot be specifi ed with precision.The bulge is embedded in a weak central bar whose long axis is oriented between 10°° and 40° from our line of sight — making it diffi cult to detect. Some researchers think there are two superposed bars.Our Milky Way Galaxy is well above average in terms of size, luminosity, and mass. For instance, our Local Gal-axy Group contains only one other galaxy of similar size (Messier 31), one that’s somewhat smaller (Messier 33), and several dozen that are much smaller.The Milky Way contains well over 100 billion stars and probably well under a trillion — most of them consider-ably less massive and much less luminous than our Sun. Its total luminosity is at least 15 billion Suns, correspond-ing to an integrated absolute magnitude of –20.5.Although stars account for almost all of our galaxy’s light, they make up only a fraction of its mass. A roughly comparable mass exists in the form of interstellar gas (mostly hydrogen and helium) and “dust” (microscopic particles). Dust accounts for less than 1% of the mass, but it has a disproportionate eff ect on our galaxy’s appear-ance and evolution because it blocks visible light, whereas hydrogen and helium are transparent at most wavelengths. The Galactic CenterAstronomers have long suspected that the center of our galaxy lies near the junction of the constellations Sagit-tarius, Scorpius, and Ophiuchus. That’s both because this is the brightest and broadest part of the Milky Way band and because of the distribution of globular clusters. Of the 29 Messier globulars, seven are in Sagittarius, three in Scorpius, and six in Ophiuchus, compared to just one (M79 in Lepus) in or near the winter Milky Way.The suspicion was confi rmed when radio observations detected a strong source at right ascension 17h 45.7m, declination –29° 00′, a visually unremarkable spot about 5° west-northwest of Gamma ( γ) Sagittarii, the western-This image of NGC 4565 illustrates why it’s diffi cult to discern the Milky Way’s structure. We see both galaxies edge-on, with all their spiral arms superposed. Moreover, most of our galaxy is blocked from view by the dust that clogs its central plane.Like the Milky Way, Messier 83 is an SBc spiral. But even with the luxury of observing it from “above,” it’s not easy to discern M83’s structure. How many spiral arms do you count? Two? Three? Five? Eight?ESO (2)Milky Way Pt1 layout.indd 265/2/13 3:28 PM
SkyandTelescope.comJuly 2013 27most star in the Teapot asterism. Several lines of evidence indicate that these radio emissions come from an accre-tion disk spiraling around a supermassive black hole at the galaxy’s center.The galactic nucleus — the area around the center — can be studied only at radio, microwave, and infrared wavelengths because the dust of the galaxy’s interior spiral arms, and the dense dust around the nucleus itself, blocks virtually all radiation at shorter wavelengths. Vis-ible light from the nucleus is obscured roughly 30 magni-tudes —a factor of one trillion — by intervening matter.The nucleus is something like an extreme globular cluster, crowding more than 100 million stars into a sphere some 150 light-years in radius. (By contrast, there are only about 50,000 stars within 150 light-years of the Sun.) Deep inside the nucleus are three compact open clusters of rapidly evolving giant and supergiant stars mingled with dense gas and dust.The Central BulgeDespite the heavy dust clouds in the central regions of our galaxy, we can see four layers of galactic structure when we look toward Sagittarius and Scorpius.The Great Sagittarius Star Cloud is the innermost galactic structure that can be observed in visible wave-lengths and the most distant Milky Way structure that can be seen with the unaided eye. It stretches several degrees north from Gamma and Delta (δ) Sagittarii and is a splendid sight in small binoculars — a bright glow with multitudes of momentarily resolved star-sparks. It is in fact a section of the Milky Way’s central bulge.The central bulge is depleted of the gas and dust from which new stars form, so unlike the spiral arms it contains no bright, young, blue stars. Instead, its bright-est stars are K-type orange giants. So on color photos the Great Sagittarius Star Cloud has a yellowish tint.Most of our galaxy’s bulge is hidden from our view by the dust of the inner spiral arms. If we could see the whole bulge, it would stretch from the Stinger of Scorpius to the Small Sagittarius Star Cloud (Messier 24) and reach Akira Fujii’s masterful photograph shows the central Milky Way as it appears from mid-northern latitudes. The dotted line marks the galactic plane. The ellipse shows how big the central bulge would appear if there was no dust in the way, assuming that it’s an 8,000 × 10,000 light-year ellipsoid centered 27,000 light-years away.M6M20M23M76231M25M17M16M8Galactic CenterGreat SgrStar CloudM24AntaresMilky Way Pt1 layout.indd 275/2/13 9:43 AM
28 July 2013sky & telescopeGalactic Depth PerspectiveThe Sagittarius-Carina Arm of our galaxy is studded with active star-forming regions. One of the brightest is Messier 16, shown here in false color with oxygen, hydrogen, and sulfur represented by blue, green, and red, respectively.more than halfway to Antares, as shown on the preced-ing page. The Great Sagittarius Star Cloud is visible only because of a rather large window through the interstellar dust of the galactic interior.Several other similar windows allow us to see relatively long distances across our galaxy’s spiral disk. Another window through which we glimpse a segment of the galaxy’s central bulge is in the direction of the foreground open cluster Messier 7 in the Tail of Scorpius. On color photos the little star cloud around M7 has the same yellowish tone as the Great Sagittarius Star Cloud to its north. Thus we see M7, roughly 1,000 light-years away, superposed on the much more distant galactic bulge.The Norma ArmAnother window through the interstellar dust of the galactic interior gives us a view of the second most dis-tant galactic structure visible in Sagittarius: the Small Sagittarius Star Cloud, also known as Messier 24. It’s a roughly rectangular glow stretching northeast to south-west, measuring about 2°°× ¾°, and located 2° north-northeast of Mu (μ) Sagittarii. In binoculars it’s not as bright as the Great Sagittarius Star Cloud, but it’s more richly sprinkled with 7th- to 10th-magnitude stars. M24’s estimated distance of 10,000 to 16,000 light-years implies that it’s a stretch of one of the deep interior spiral features of our galaxy. Its binocular appearance confi rms this, because spiral arms have many highly luminous super-giants that should be easily resolvable in small instru-ments even at M24’s distance.M24 is probably part of the Norma Arm, the second spiral arm in from our own Orion-Cygnus Arm. Some researchers call this the Scutum-Centaurus Arm and use the term Norma Arm for a smaller arm that lies closer to the galactic center. This is the terminology adopted in the galaxy diagram on the foldout Milky Way map in the center of this magazine.The bright Norma Star Cloud in the far southern Milky Way and the Scutum Star Cloud north of Sagittar-ius probably also lie in the Norma Arm. We will discuss them in future articles.The Sagittarius-Carina ArmMoving outward from the Norma Arm toward the Sun, the next spiral feature is the Sagittarius-Carina Arm — so named because many major bright emission nebulae and open clusters are distributed along it from Sagittarius to Carina. From northeast to southwest these include M16 (the Star Queen or Eagle Nebula) in Serpens; the emission nebulae M17 (Swan), M20 (Trifi d), and M8(Lagoon) in Sagittarius; the open clusters M21 in Sagit-tarius and NGC 6231 in Scorpius; the open cluster and emission nebula NGC 6193 and 6188 in Ara; the open cluster NGC 4755 (Jewel Box) in Crux; and the giant emission nebula NGC 3372, also known as the Eta Cari-nae Nebula. All of the emission nebulae contain embed-ded clusters and/or associations of young stars, though in some cases these stars are heavily obscured by dust.Although it’s rather far south for observers in the northern United States and Europe, NGC 6231 in the Tail of Scorpius, a major tracer of the Sagittarius-Carina Arm, is well worth viewing in binoculars. It’s a special cluster, one of the richest concentrations of extremely hot and luminous O-type giants and supergiants known in our galaxy. An outlying member of the cluster is the 4.8-mag-nitude star Zeta1 (ζ1) Scorpii, ½° to its south. Zeta1 is a B1.5 Ia+ extreme supergiant with an absolute magnitude around –8.8 (a luminosity of almost 300,000 Suns).Centered about 1° north-northeast of NGC 6231 is a rich binocular fi eld of 6th- to 9th-magnitude stars that is cata-logued as the open cluster Collinder 316 or Trumpler 24: it’s in fact the richest outlying part of Scorpius OB1, the vast stellar association of which NGC 6231 is the core. This area is also discussed in Binocular Highlights on page 40.The Sagittarius-Carina Arm clusters and nebulae in Sagittarius and Scorpius are between 4,500 and 7,000 light-years distant, suggesting that in this direction the arm is centered about 5,500 light-years from us. Further northeast along the arm, M16 in Serpens is somewhat more distant, about 6,500 light-years. Here the Sagit-tarius-Carina Arm begins its arc in toward the galactic interior. The arm’s incurving edge is at the Scutum Star Cloud, where the Sagittarius-Carina and Norma arms may intersect. On the opposite, southwestern end of the Sagittarius-Carina Arm, the Eta Carinae complex is about TRAVIS RECTOR / BRENDA WOLPA / NOAO / AURA / NSFMilky Way Pt1 layout.indd 285/2/13 9:43 AM
SkyandTelescope.comJuly 2013 29The region around Antares is where the bright stars of the Scorpius-Centaurus Association overlap the dust clouds of the Great Rift. The stars illuminate the clouds and also cause them to fl uoresce, yielding a riot of color in deep photographs. Note the globular cluster Mess-ier 4 and the contrast between the red supergiant Antares and the blue star Sigma (σ) Scorpii.8,000 light-years away. But other Sagittarius-Carina Arm associations have been identifi ed beyond it, because here the arm is beginning to curve out to the galactic exterior and we have a long view down its outcurving length.The Orion-Cygnus ArmThe fourth feature of galactic structure visible toward Sagittarius is of course our own Orion-Cygnus Arm. Its inner edge is marked by the Great Rift chain of dust clouds from Deneb on the northeast to Alpha Centauri on the southwest. The Great Rift will be described in more detail in the next installment of this article.The inner edge of the Orion-Cygnus Arm is also traced by the Scorpius-Centaurus Association, which includes the majority of the bright stars from Scorpius on the north-east, through Lupus and Centaurus, to Crux on the south-west. This association is centered about 550 light-years away in a direction between Alpha (α) Lupi and Zeta Centauri, and is highly elongated, being 700 light-years long, 250 light-years “high” (perpendicular to the galactic plane), and 400 light-years deep along our line of sight. It’s slightly nearer than the Great Rift chain of dust clouds, though Antares and a couple of other stars in the Head and Heart of Scorpius are just within the nearest fringes of Great Rift dust. Except for the M-type red supergiant Antares, all the bright stars of the Scorpius-Centaurus Association are blue B0, B1, and B2 main-sequence and giant stars. In binocu-lars the color contrast between ruddy-orange Antares and the silver-blue Sigma (σ) and Tau (τ) Scorpii to its west-northwest and southeast is absolutely stunning. All three stars fi t in the same binocular fi eld of view.Binoculars also show the bright (5.7-magnitude) globu-lar cluster M4 just 1.3° west of Antares. In 10×50 glasses, M4 appears as a large hazy patch (June issue, page 45). It’s one of the two or three nearest globulars to us, roughly 7,200 light-years away. This is about the same distance as the Sagittarius-Carina Arm, but globular clusters are far too old to be true spiral arm tracers. Spiral arms are thought to change fairly rapidly, perhaps even disappear-ing and reforming over billion-year time frames. Most of the Milky Way’s globular clusters, by contrast, seem to be nearly as old as the galaxy itself.Moreover, M4 is a good way off the Sagittarius-Carina Arm, about 16° northwest of its core. So observers in M4 would have an excellent view “down” into the Sagittarius-Carina Arm. And because M4 is well outside the dust clouds that lie along the center of the Milky Way, any observers in the cluster would have a far better view than we do of the galaxy’s central bulge.In the next two articles of this series we will visit our galaxy’s cardinal points to further our sense of depth per-spective on our place in the Milky Way Galaxy. ✦Craig Crossen is a professional editor living in Vienna, Austria. He is coauthor with Gerald Rhemann of the book Sky Vistas: Astronomy for Binoculars and Richest-Field Telescop

深空天体分类统计

距离以内的深空数量统计，超过60已经到达比较远的河外了。45-60kpc是大麦哲伦星云内的天体。小麦哲伦呢？很遗憾，测量了距离的天体并没有小麦哲伦云里面的。基本上30kpc以内就属于近河内了。

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